ARM: dts: aspeed: ast2500evb: Enable built in RTC

Enable this device so the RTC driver can be tested on the EVB.

Signed-off-by: Joel Stanley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/arm/boot/dts/aspeed-ast2500-evb.dts

···
      status = "okay";
      memory-region = <&gfx_memory>;
 };
+
+&rtc {
+	status = "okay";
+};